The present application relates to a battery using a carbon material as an anode active material.
In recent years, downsizing and weight saving of portable electronic devices represented by a mobile phone, a PDA (Personal Digital Assistant), and a notebook personal computer have been actively promoted. As a part thereof, improving the energy density of a battery as a driving power source for such electronic devices, in particular, the energy density of a secondary battery has been strongly desired.
As a secondary battery capable of providing a high energy density, for example, the secondary batteries using lithium (Li) as an electrode reactant are known. Specially, the lithium ion secondary batteries using a carbon material capable of inserting and extracting lithium for the anode are widely used practically. However, in the lithium ion secondary battery using the carbon material for the anode, the technique has been already developed to the degree close to its theoretical capacity. Therefore, as a method to further improve the energy density, it has been considered that the thickness of the active material layer is thickened to increase the ratio of the active material layer in the battery, and the ratios of the current collector and the separator are lowered (for example, refer to Japanese Unexamined Patent Application Publication No. 9-204936).
However, when the thickness of the active material layer is increased without changing the volumetric capacity of the battery, the area of the current collector is relatively decreased. Therefore, when charged, the current density to the anode is increased. In the result, capacities of diffusion of lithium ions and electrochemical reception of lithium ions in the anode fall short, and thus metal lithium is easily precipitated. The metal lithium precipitated in the anode as above is easily deactivated, leading to enormous lowering of the cycle characteristics. Consequently, it has been difficult to increase the thickness of the active material layer.